Reverse scans

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

Pitting and Crevice Corrosion The general literature for pre-dic ting pitting tendency with the slow scan reviews the use of the reverse scan if a hysteresis loop develops that comes back to the repassivation potential below the FCP (E ) the alloy will pit at... 

Cyclic polarisation This type of measurement is similar to potentio-dynamic anodic polarisation with the difference that, following an anodic polarisation plot, the test specimen is subjected to a cathodic stimulus, i.e. a reverse scan. Any hysteresis, i.e. deviation from the anodic plot, can... 

FIGURE 2-3 Concentration distribution of the oxidized and reduced forms of the redox couple at different times during a cyclic voltammetric experiment corresponding to the initial potential (a), to the formal potential of the couple during the forward and reversed scans (b, d), and to the achievement of a zero reactant surface concentration (c). 

Cyclic voltammetry is most commonly used to investigate the polymerization of a new monomer. Polymerization and film deposition are characterized by increasing peak currents for oxidation of the monomer on successive cycles, and the development of redox waves for the polymer at potentials below the onset of monomer oxidation. A nucleation loop, in which the current on the reverse scan is higher than on the corresponding forward scan, is commonly observed during the first cycle.56,57 These features are all illustrated in Fig. 3 for the polymerization of a substituted pyrrole.58... 

Such effects are observed inter alia when a metal is electrochemically deposited on a foreign substrate (e.g. Pb on graphite), a process which requires an additional nucleation overpotential. Thus, in cyclic voltammetry metal is deposited during the reverse scan on an identical metallic surface at thermodynamically favourable potentials, i.e. at positive values relative to the nucleation overpotential. This generates the typical trace-crossing in the current-voltage curve. Hence, Pletcher et al. also view the trace-crossing as proof of the start of the nucleation process of the polymer film, especially as it appears only in experiments with freshly polished electrodes. But this is about as far as we can go with cyclic voltammetry alone. It must be complemented by other techniques the potential step methods and optical spectroscopy have proved suitable. 

The situation is different on GC where only one irreversible cathodic wave is observed at —1.6 V, associated with disintegration of the compound, and one anodic wave on the reverse scan, caused by stripping of the lead released by the reduction ... 

The double-layer charging current thus tends toward a plateau equal to Cdv with a rise time equal to RuCd (Figure 1.7). On the reverse scan,... 

Trace crossing upon scan reversal appears for intermediate values of X (Figure 2.9). It results from the fact that the C concentration continues to build up during the forward and reverse scans. More C has thus been formed... 

During the reverse scan, the Laplace transformation is based on t-t as the time variable. Equation (1.10) then becomes... 

The unsymmetrical shapes of the forward and reverse components of an SW voltammogram have similar origins as those of the CV. However, unlike the reverse scan wave in CV, the reverse SWV wave is measured almost simultaneously with the forward component. Therefore, there is much less accumulation of the reaction product at the electrode surface during the potential scan. This feature of SWV makes it very useful for understanding... 

Because JPS is limited by reaction kinetics and mass transport a dependency on the HF concentration cHf and the absolute temperature Tcan be expected. An exponential dependence of JPS on cHf has been measured in aqueous HF (1% to 10%) using the peak of the reverse scan of the voltammograms of (100) p-type electrodes. If the results are plotted versus 1/7) a typical Arrhenius-type behavior... 

Actually, if we look at the cyclic voltammogram of a non-aqueous solution of [Rh4(CO)i2] shown in Figure 2, we see a reduction profile (peak A) which lacks a directly associated re-oxidation peak in the reverse scan. 

In agreement with such an electronic distribution, the cyclic voltammogram of ferrocene displays an oxidation profile (peak A) which is accompanied in the reverse scan by a directly associated reduction process (peak B), Figure 4. 

As Figure 21 shows, even closer are the two oxidations in triferrocenylphosphate, [O = P 0(C5H4)Fe(C5H5) 3]. The small spurious peak present at about + 0.2 V in the reverse scan of the cyclic voltammogram is likely due to electrode adsorption phenomena, given the ascertained stability of the corresponding trication.34... 

As a typical example, we consider the behaviour of the chloro-hydride complex [RuHC1(PP3)], which displays a rather simple conversion mechanism. In fact, Figure 16a shows that it undergoes an irreversible two-electron oxidation that generates, on the reverse scan, a voltam-metric profile identical to that observed for the chloro monocation [RuC1(PP3)] +, Figure 16b. 

Figure 19 shows that the irreversible (one-electron) oxidation of [Co(H2)(PP3)j+ generates [CoH(PP3)]+. In fact, the voltammetric profile which appears in the reverse scan of Figure 19a is perfectly complementary to that of the oxidation of the neutral complex [CoH(PP3)], Figure 19b. 

The Rh(I)-dimethylfumarate complex undergoes an irreversible (two-electron) oxidation, that generates in the reverse scan a cathodic profile completely identical to that of the Rh(III)-succinyl derivative, Figure 21a. 

In fact, the latter displays a first (one-electron) reversible reduction followed by a second irreversible reduction that in turn shows, on the reverse scan, the oxidation peak of the Rh(I)-dimethylfumarate complex, Figure 21b. The complete interconversion process is illustrated in Scheme 10. 

In these techniques, the concentrations at the electrode do not immediately attain their extreme values after the start of the experiment. Rather, they change with E ox t according to equation (1). While the steepness of the concentration profiles increases with E (forward scan), simultaneously 8 increases in the quiet solution. The latter effect slows down the increase of i with E, and finally (close to the limiting current region) leads to the formation of a peak with a characterishc asymmetric shape. On the reverse scan (after switching the scan direction ad. Ef), products formed in the forward scan can be detected (B, in the case discussed). 

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